Z2

X v

FIGURE 3-6. Translocation. Karyotype of a patient with a translocation between the long arms of chromosomes 5 and 6, as identified by the arrows. The break on the chromosome 5 is at band q13 and that of chromosome 6 is q23. This patient is described as has 46,XY,t(5;6)(q13;q23). (Courtesy of Dr. R.S. Sparkes.)

FIGURE 3-7. Robertsonian translocation. Karyotype of a male carrier of a Robertsonian translocation involving chromosomes 13 and 14. This patient has 45 chromosomes and is described as having 45,XY,t(13q,14q). (Courtesy of Dr. R.S. Sparkes.)

Down's syndrome (trisomy 21) are caused by Robertsonian translocations. Carriers of reciprocal translocations are usually clinically normal and are detected because of unbalanced offspring; such carriers may produce a child with multiple anomalies or have a history of fetal wastage.

A nomenclature system has been adopted to describe the human chromosome complement and indicate departures from normalcy. An extra or missing chromosome or a piece of a chromosome is indicated by a plus ( + ) or a minus ( — ), respectively; thus, 47,XX+21 is a female with trisomy 21. A female with a partial trisomy of the long arm of chromosome 3 would be described as 46,XX,3q+. A female with an interstitial deletion of the long arm of one chromosome 13 would be depicted as 46,XX, 13q— (see Fig. 3-4). A male with a translocation in whom DNA has been transferred from the long arm of one chromosome 5 with a breakpoint at band 13 to the long arm of one chromosome 6 with a breakpoint at band 23 and visa versa without apparent loss of DNA would be described with a "t" as 46,XY,t(5;6)(q13;q23) (see Fig. 3-6). The ring chromosome is depicted as "r," and the structure may be unstable. For example, a ring of chromosome 11 would be described as 11r (Fig. 3-5).

Chromosomal analysis is required for definitive diagnosis of any anomaly. Case reports of familial translocations, deletions, and duplications in which identification of structural abnormalities using chromosomal banding techniques have led to the delineation of many syndromes of partial monosomy (deletion) or partial trisomy (duplication).

Many identifiable chromosomal syndromes exhibit ocular manifestations, and the most common include hypertelorism, epicanthus, up- or downward slanting of the palpebral fissures, blepharoptosis, strabismus, and microphthalmia. However, these features are not specific, and any or all structures of the eye may be abnormal in a patient with a chromosomal anomaly. The most common manifestation with the potential for visual impairment is microphthalmia, a malformation in which the anteroposterior diameter and volume of the eye are reduced; it is a relatively common malformation, occurring in 0.22 in 1000 live births, and has been reported to be associated with a variety of chromosomal anomalies.148 A coloboma of the uvea (iris in or choroid in Fig. 3-8) is caused by incomplete closure of the fetal fissure; the "typical" position is inferior to nasal. Incomplete closure of the fetal fissure can cause a spectrum of malforma-

FIGURE 3-8. Coloboma of the iris (A) and of the choroid/retina (B).

tions ranging from an iris coloboma or microcornea to clincial anophthalmia. The prevalence of coloboma of the eye has been reported to be 0.26 per 1000 births.148 Any chromosomal defect associated with a colobomatous ocular malformation should be considered to be associated with a malformation within this spectrum. Table 3-1 summarizes the chromosomal anomalies that have been reported in association with colobomatous and

TABLE 3-1. Chromosomal Aberrations in Colobomatous Microphthalmia.

Duplications

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